Prolonging cardiac action potential and thus the effective refractory period by inhibiting K channels may provide an important antiarrhythmic (class III) action in the setting of reentrant arrhythmias. Currently available class III drugs are effective in less than 30 or 40% of the patients, and become proarrhythmic under certain conditions. To design better class III drugs, information about the molecular mechanisms of drug actions is needed. The conventional approaches employ cardiac tissues or myocyte to study drug effects on native K channels. Although much about drugs' blocking potencies can be learned, these approaches are limited by the difficulties of single channel analysis, and the inability to specifically modify channel structure and examine the resultant changes in drug actions. In this application, we propose a new approach: studying drugs actions on K channel clones from cardiac libraries. Three types of channels will be studied: transient outward (Kvl.4), rapid delayed rectifier (Kvl.2) and slow delayed rectifier (IsK). We will perform detailed kinetic analysis of drug-channel interactions at both whole-cell and single-channel levels. More importantly, with information about the structure-function relationship of these channels available and the ability to specifically mutate their sequences, we can now address the issue of molecular mechanisms of drug actions. We will focus on 3 drugs: quinidine and clofilium block all three channels but with qualitative and quantitative differences in their actions; proquil blocks IsK specifically. We have formulated hypotheses about the mechanisms of drug actions and will test them in the proposed experiments. Another important question regarding anti arrhythmic or proarrhythmic activities of drugs is how their actions may be modified by diseased conditions such as ischemia and infarction. Since the actions of antiarrhythmic drugs depend on their substrates and the mechanisms of arrhythmogenesis, it is important to study drug actions under abnormal conditions. In this application we will study how perturbations in ionic composition and activation of PKA and PKC likely to occur during ischemia and infarction can modulate the function of Kv1.4, Kv1.2 and IsK, and their responses to K channel blockers. Our approach of combining electrophysiological and molecular biological techniques will generate more exact information abut the mechanism of drug actions and how they are modified by diseased conditions, and thus may aid the design of new drugs or better usage of currently available drugs.